Sticky header, dim overlay on card navigation, fix sticky stacking

Make header sticky so search button stays visible on scroll. Section
headers stick below it using a JS-measured --header-height variable.
Add dim overlay behind highlighted cards for better focus effect.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/js/app.js

@@ -427,6 +427,18 @@ document.addEventListener('DOMContentLoaded', async () => {
     // Show content now that translations are loaded and tabs are set
     document.body.style.visibility = 'visible';
 
+    // Set CSS variable for sticky header height so section headers stack below it
+    const headerEl = document.querySelector('header');
+    if (headerEl) {
+        const updateHeaderHeight = () => {
+            document.documentElement.style.setProperty(
+                '--header-height', headerEl.offsetHeight + 'px'
+            );
+        };
+        updateHeaderHeight();
+        window.addEventListener('resize', updateHeaderHeight);
+    }
+
     // Initialize command palette
     initCommandPalette();